Mechanisms of the transduction of light to specific ionic permeability changes are being studied in the large photoreceptor of the barnacle B. eburneus, and in the giant cell of the abdominal ganglion of Aplysia California. Techniques being used are: 1) Voltage clamp, 2) ion-specific electrodes, 3) intracellular injection, 4) spectrophotometry of pigment changes, and 5) fluorescence changes of molecular probes. Studies are also being conducted to determine the specific membrane mechanism of ion permeation in these preparations by determining selectivity sequences of mono- and divalent cations and anion series.